Cross-Talk Suppression in Geiger-Mode Avalanche Photodiodes

ABSTRACT

An avalanche photodiode detector is provided with a substrate including an array of avalanche photodiodes. An optical interface surface of the substrate is arranged for accepting external input radiation. There is provided at least one cross-talk blocking layer of material including apertures positioned to allow external input radiation to reach photodiodes and including material regions positioned for attenuating radiation in the substrate that is produced by photodiodes in the array. Alternatively at least one cross-talk blocking layer of material is disposed on the optical interface surface of the substrate to allow external input radiation to reach photodiodes and attenuate radiation in the substrate that is produced by photodiodes in the array. At least one cross-talk filter layer of material can be disposed in the substrate adjacent to the photodiode structures, including a material that absorbs radiation in the substrate that is produced by photodiodes in the array.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims the benefit of U.S. Provisional Application No.61/214,934, filed Apr. 30, 2009, the entirety of which is herebyincorporated by reference.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH

This invention was made with Government support under Contract No.FA8721-05-C-0002 awarded by the Department of Defense. The Governmenthas certain rights in the invention.

BACKGROUND OF INVENTION

This invention relates generally to electronic photodiodes, and moreparticularly relates to avalanche photodiodes such as Geiger-modeavalanche photodiodes.

Avalanche photodiodes (APDs) operating in Geiger-mode (GM) can beemployed to detect single infrared photon arrival with sub-nanosecondaccuracy. As a result, Geiger-mode avalanche photodiode arrays arereceiving increased interest for a number of photon-countingapplications, including astronomy, three-dimensional laser radar(LADAR), and photon-counting based optical communication.

In Geiger-mode operation, an avalanche photodiode is biased above itscharacteristic breakdown voltage. This is a metastable state because thegeneration of an electron-hole charge pair in the photodiode, eitherthermally or through absorption of a photon, can cause the photodiode tobreak down. For example, upon absorption of a photon at a thusly biasedphotodiode, breakdown produces a rapid rise in current, which ultimatelybecomes limited by series resistance and internal space-charge effects.Because an avalanche photodiode when operated in Geiger mode isinitially biased a few volts above breakdown, the breakdown event causedby photon absorption produces a large voltage signal swing that issufficient for directly driving CMOS digital logic.

This is an important attribute of Geiger-mode APDs and has allowed thedevelopment of Geiger-mode arrays bonded directly to readout integratedcircuits (ROICs) and micro-optics to form focal plane arrays for use inimaging or other applications. The ability to produce arrays ofphotodiodes and read them out at high data rates is important for bothLADAR and optical-communications applications. The use of an all-digitalreadout reduces power, and makes the APD technology more easily scalableto large array sizes than competing technologies employing, e.g.,linear-mode APDs or photomultiplier tubes.

One limitation of such densely packed Geiger-mode APDs arrays is opticalcross-talk. When operated in or near Geiger-mode, avalanche photodiodesgenerate many highly energetic electron/hole charge carrier pairs. Someof these charge carriers lose energy by emitting within the photodiodeitself a spectrum of photons, which can be detected at other nearbyphotodiodes in an array of photodiodes. Such detection of photons thatare secondary, i.e., produced at and coming from a neighboringphotodiode rather than from a source external to the photodiode array,cause corresponding secondary detection events across the photodiodearray. Cross-talk is the term used herein to describe this process ofsecondary photon detection across an APD array. As Geiger-mode APD arraysize, density, and performance requirements increase, optical cross-talkbecomes an increasingly limiting source of such secondary photondetection.

SUMMARY OF THE INVENTION

The invention provides photodiode detector designs, and processes forfabricating such designs, that substantially suppress cross-talk effectsin an array of avalanche photodiodes. In one example there is providedan avalanche photodiode detector with a substrate including an array ofavalanche photodiodes. An optical interface surface of the substrate isarranged for accepting external input radiation. In one example, thereis provided at least one cross-talk blocking layer of material thatincludes apertures positioned to allow external input radiation to reachphotodiodes and that includes material regions positioned forattenuating radiation in the substrate that is produced by photodiodesin the array. In a further example, there is provided at least onecross-talk blocking layer of material, disposed on the optical interfacesurface of the substrate, that allows external input radiation to reachphotodiodes and that attenuates radiation in the substrate that isproduced by photodiodes in the array. In a further example, at least onecross-talk filter layer of material is disposed in the substrateadjacent to the photodiode structures. This cross-talk filter layerincludes a material that absorbs radiation in the substrate that isproduced by photodiodes in the array.

With these configurations, the photodiode detector designs of theinvention enable highly efficient photon detection that is increasinglyrequired for modern astronomy, three-dimensional laser radar (LADAR),and photon-counting based optical communications. Other features andadvantages of the invention will be apparent from the followingdescription and accompanying figures, and from the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic cross-sectional sideview of a portion of an APDdetector array stack;

FIG. 2 is a schematic diagram of a control circuit for arming,quenching, and digitally encoding photon arrival times for the APDdetector array stack of FIG. 1;

FIG. 3 is a schematic cross-sectional sideview of the APD detector arraysubstrate of FIG. 1, here showing the optical path of cross-talk photonsgenerated by an avalanche event at a first photodiode;

FIGS. 4A-4C are schematic views of selected photon paths taken from FIG.3;

FIG. 5 is a schematic perspective view of an APD detector arraysubstrate including a back side cross-talk blocking layer forattenuating optical cross-talk in the substrate;

FIG. 6 is a schematic sideview of the APD detector array substrate andback side absorber layer of FIG. 5, here showing the optical path ofcross-talk photons that are absorbed by the back side cross-talkblocking layer;

FIG. 7 is a plot of the probability of causing two or more cross-talkevents integrated over a 9×9 photodiode neighborhood as a function ofphotodiode over bias for an APD detector array without a back sidecross-talk blocking layer and for an APD array that included a back sidecross-talk blocking layer of TiAu;

FIGS. 8A-8B are plots of the integrated cross-talk probabilities over a9×9 photodiode neighborhood as a function of photodiode over bias for anAPD detector array without a back side cross-talk blocking layer and foran APD detector array that included a back side cross-talk blockinglayer of TiAu, respectively, as well as the corresponding backgroundnoise;

FIG. 9 is a plot of the optical reflection percentage as a function ofincident angle for GeCr and TiAu back side cross-talk blocking layerslike that of FIG. 5;

FIG. 10 is a plot of the optical reflection percentage as a function ofwavelength for four GeCr back side cross-talk blocking layers like thatof FIG. 5 and having differing Cr thicknesses;

FIG. 11A is a schematic cross-sectional side view of an APD detectorarray stack including both a back side cross-talk blocking layer likethat of FIG. 5 and bulk cross-talk blocking layers;

FIG. 11B is a schematic cross-sectional side view of an APD detectorarray stack including bulk cross-talk blocking layers like that of FIG.11A and an APD array substrate that is bonded to a microlens array;

FIG. 12 is a schematic side view of two photodiodes in an APD detectorarray that includes a spectral filter for attenuating cross-talk photonsproduced by avalanche events at the photodiodes and that includes a backside cross-talk blocking layer for attenuating cross-talk photonsproduced by avalanche events;

FIG. 13 is a plot of an example characteristic APD detector avalanchephoton emission spectrum;

FIG. 14 is a plot of measured cutoff wavelengths of an APD absorberlayer and two spectral filters designed for APD detector operation near1550 nm;

FIG. 15 is a plot of design specification for wavelength cutoff of anabsorber layer and of a spectral filter layer for the APDs of FIG. 12;

FIG. 16 is a plot of normalized quantum efficiency as a function ofwavelength for APDs designed with and without a spectral filter layerlike that of FIG. 12;

FIG. 17 is a plot of the spectral response to cross-talk avalancheemission for an APD detector array with and without a spectral filterlayer like that of FIG. 12, and is a plot of the avalanche emissionspectrum;

FIG. 18 is a plot of the integrated cross-talk probability over a 9×9photodiode neighborhood as a function of photodiode over bias for an APDdetector array with a back side absorber layer and with four differingabsorbers and spectral filter layers; and

FIGS. 19A-19H are schematic sideviews of an APD detector array substrateand sacrificial platform as bulk cross-talk blocking layers, APD layers,and a microlens array are fabricated.

DETAILED DESCRIPTION OF THE INVENTION

Referring to FIG. 1, there is schematically shown a region of a focalplane array 10 of an avalanche photodiode detector, in sectionalsideview. The array is provided in this example as a stack of threecomponents including a substrate 12 that supports an array of avalanchephotodiodes (APDs) such as the three example photodiodes 14, 16, 18shown in the figure. Each photodiode is in this example configurationformed as a structure of photodiode layers, e.g., as an inverted mesastructure, and can include, e.g., an absorber layer, a grading layer, afield-stop, or charge, layer, and an avalanche, or multiplier, layer,identified together schematically in the figure as 20, 22, 24, for thephotodiodes 14, 16, 18, and described in detail below. An avalanchephotodiode structure of this configuration is commonly referred to as aSAM or SAGCM structure.

The APD detector focal plane array stack includes a microlens array 30that is positioned adjacent to the APD detector array substrate 12 andcan be separated from the substrate by an air gap 31 as described indetail below. The microlens array focuses incoming light 38, i.e.,photons, and directs the cone 40 of focused light to a correspondingphotodiode 22. The extent of the gap 31 is accordingly set to optimizethe coupling of light from the lenses to the APDs by matching the lensfocal length to the thickness of the focal plane array stack. Without amicrolens, each photodiode could be sensitive to external light incomingto the array from any angle; the microlens array excludes suchbackground optical input from the APD array. In addition, by focusingincoming light, the microlens array enables a reduction in the requiredphotodiode size, thereby increasing the radiation hardness and reducingthe dark count rate and optical cross-talk of the array.

The focal plane array further includes a semiconductor platform, such asa microchip, that provides a circuitry such as a photodiode read outintegrated circuit (ROIC) 48, that is connected to the APD array toproduce electrical signals indicative of photon arrival time stamps,and/or other information, for each photodiode in the array. Bump bonds42, 44, 46, or other suitable connection technologies as are known inthe art, are provided to connect the photodiodes of the array to thecircuitry of the ROIC 48. Not shown in the figure is packaging of thefocal plane array stack and stack cooling devices provided, as arecommonly employed, for cooling the stack during operation.

Referring also to FIG. 2, in operation of the APDs of the focal planearray detector as Geiger-mode avalanche photodiodes, each photodiode 14is separately armed by an arming circuit 25, at a bias above thebreakdown voltage for detecting an incoming photon 40, by an arm pulse26. When an incoming photon 40 is directed to a photodiode 14 of thearray and an avalanche breakdown event occurs, an arming capacitor 28discharges through current limiting resistance of the photodiode. Atsome point during the avalanche breakdown event, depletion of theavailable charge restricts the APD discharge, which follows anexponential decay, until the digital timing logic 27 registers theavalanche event as detection of an incoming photon and engages an activequench controller 29, if such is desired for a given application, toquench the event.

In one example configuration for registering avalanche events, thedigital timing logic 27 times photon arrivals at each photodiode intodiscrete time bins, so that the position and time of each detectionevent across the APD array can be indicated. The ROIC 48 can beconfigured to correspondingly operate in a framed mode for thephotodiode bins, whereby the entire photodiode array is armed together,allowed to observe for a time, and then disarmed, with each photodiodetime bin then digitally read out to a buffer, and then all photodiodesre-armed together. With this arrangement, once a photodiode commences anavalanche event due to an incoming photon, that photodiode cannotregister a second avalanche event within the remaining duration of aframe and is insensitive to any further arriving photons. This exampleAPD array control can be particular advantageous for many photoncounting applications, but is not in general required by the invention.Any suitable APD array control technique can be employed for setting thebias and sensing the avalanche events of photodiodes in the detection ofincoming photons.

Whatever photodiode control methodology is employed, when a photonincoming to the APD array reaches a photodiode and causes an avalancheevent at that photodiode, broad spectrum secondary light is produced bythe photodiode in the photodiode multiplication layer during theavalanche process. Referring to FIG. 3, this secondary light 50propagates outward from the photodiode 14 at which the light wasproduced, back through the APD array substrate 12. Any of this secondarylight that is directed through the substrate toward the back surface 51of the substrate can reflect off of that back surface and propagate backinto the substrate 12 at a corresponding reflection angle.

Depending on the geometry of the APD array, the size and spacing of eachphotodiode active area in the array, and the thickness of the APD arraysubstrate, secondary light 52, 54, 56 that reflects off of the backsurface of the substrate may be directed to photodiodes 16, 18 in theneighborhood of the photodiode 14 at which they originated. Thissecondary light in turn may cause avalanche events at the neighboringphotodiodes at which they arrive, if the neighboring photodiodes arearmed for photon detection, triggering such neighboring photodiodes togenerate spurious photon count signals that are indicative of secondaryphoton detection rather than primary external photon detection.

Referring also to FIGS. 4A-4C, this process is shown in another view ofthe APD array, with an incoming photon 40 directed at a first photodiode14, causing that photodiode 14 to trigger a detection signal and producesecondary photons 50, one of which 52 is directed to a neighboringphotodiode 16. As a result, avalanche triggering of both the neighboringphotodiode 16 as well as the primary photodiode 14 occurs. Thisavalanche triggering of photodiodes by secondary photons that areemitted by a neighboring photodiode during a primary avalanche event isreferred to as optical coupling or optical cross-talk.

As mentioned above, the degree of optical coupling between photodiodesin an array depends on the geometry of the array, the size and spacingof the active area of each photodiode, and the thickness of the APDarray substrate. An additional factor is the angle of total internalreflection that is characteristic for the interface at the front opticalinterface, or back side 51, of the APD array substrate 12, given thematerial of the substrate and the presence of a lower dielectric gapbetween the APD array substrate and the microlens. For example, given anAPD array substrate of InP and the inclusion of an air gap between thesubstrate and the microlens, the characteristic angle of total internalreflection is 17.7°. This relatively small angle for total internalreflection strongly increases the optical coupling efficiency fornext-nearest-neighbor and farther photodiodes in the APD array. Opticalanalysis, e.g., employing optical design software, can be conducted tocalculate the precise optical coupling for all photodiodes in an array.

In one technique in accordance with the invention for reducing suchoptical coupling, spatial elements, i.e., physical structural elements,are incorporated into the APD array and/or photodiode structure tomanipulate the optical path between photodiodes in the array. In oneexample of this technique, one or more characteristics of interfaces ofthe APD array substrate, e.g., the front optical interface that is inthe example here the back side surface 51, of the APD substrate 12, arecontrolled to at least partially suppress internal reflection ofsecondary photons that are produced and directed to those substrateinterface surfaces as the result of a primary avalanche event at aphotodiode in the array.

Referring to FIG. 5, there is shown one example configuration of such,here provided as one or more layers 60 of material disposed on the frontoptical surface 51 of the APD array substrate 12, which is the back sideof the substrate, in a pattern that includes open, exposed surfaceregions 62 for accepting input radiation at each photodiode such as 14,16, 18 and that includes surface regions 64 which cover the substrate 12with the material. The microlens array 30 is not shown in this view forclarity; it is to be understood that the microlens array is positionedatop the layers 60, either spaced-apart or adjacent to the layers in themanner described below.

The blanket-coating surface regions 64 of the layer 60 at leastpartially suppress internal reflection of secondary radiation that isemitted due to recombination or relaxation of charges in each photodiodestructure during an avalanche event, while the apertures 62 are providedwith appropriate geometry and spacing to allow efficient transmission ofprimary photons within the input beam path through the microlens arrayto photodiodes in the APD array.

In one configuration, the one or more layers 60 are at least partiallyabsorbing media selected to reduce the contribution of secondaryinterface reflections in the APD array substrate. Examples of suchabsorbing media are semiconductors, metals, dielectrics, polymers,molecular or atomic species, quantum wells, photonic crystals, and othersuitable media. The layer 60 can include any number of materials orsurface properties provided in any arrangement that results in a desiredsurface characteristic. For example, anti-reflection coatings and/orsimilar suitable anti-reflection surface treatments can be provided onregions of the APD array substrate back surface that are outside theoptical path of primary photons.

For a material layer to be provided on the back surface, appropriateselection of the materials to be included in the layer 60 for a givenAPD array in general requires determination of the absorption strengthof the media for the layer thicknesses that can be accommodated by theAPD array configuration as well as fabrication process. Operation of theback side absorber media layer 60 is required at both below and abovethe critical angle for total internal reflection inside the APD arraysubstrate, and thus attention is preferably also given to the angularand polarization dependence of absorption of the media. It is found thatevanescent coupling strength of the absorber media layer 60 can be, ingeneral, different than travelling wave attenuation by the media layer60 and that such evanescent coupling dominates attenuation of reflectionbetween photodiodes. Thus, depending on the spectral width andoriginating location of cross-talk photons, various different absorbingmedia can be utilized singly as the back side layer 60 or in combinationas a stack of layers.

The material layer 60 can be any suitable material or combination ofmaterials that in some way disrupts reflection of radiation from thematerial, e.g., by attenuating radiation impinging the material. Forexample, a lossy material can be employed, such as titanium, germanium,and combinations of such with, e.g., gold and chromium or other suitablematerial. But as explained above, the material can be provided as anorganic as well as inorganic material, and as a semiconductor, a metal,or an insulator. Any material that is a high-efficiency absorber but apoor radiator at the photodiode wavelength of interest, as the materialrelaxes after radiation absorption, can be a particularly goodselection. Where a semiconductor material is employed, in one examplethe semiconductor is provided as an indirect-bandgap semiconductor thatdoes not radioactively recombine when the semiconductor absorbs light atthe wavelength of interest. Any suitable indirect bandgap semiconductorcan here be employed. Alternatively, direct bandgap semiconductors canalso be selected such that radiative recombination occurs at wavelengthslonger than the cutoff wavelength of the APD absorber.

The pattern provided in the material layer 60 is preferably based on thegeometry of the APD array. For example, the geometry of the apertures 62in the layer can be selected based on the distribution of light to bedetected. For a Gaussian distribution, circular openings can bepreferred for capturing light to be detected by the photodiodes, whilefor a non-Gaussian distribution, an elliptical, oval, or other aperturecan be preferred. Any aperture geometry can be employed and more thanone aperture geometry can be employed across the pattern of apertures.Whatever aperture geometry is employed, the apertures are provided as apattern in a blanket coating of the material layer 60.

FIG. 6 is a side cross-sectional view of the APD array substrate 12 withan absorbing material layer 60 provided on the surface 51 at which lightis incoming to the array. The microlens array is not shown here forclarity. An antireflective coating 53 can be provided on the incomingoptical surface 51 and on the microlens surfaces as explained above;this coating 53 was not shown in FIG. 5 for clarity. Given a primaryavalanche event at a first photodiode 14, broad spectrum radiation 66,68, 70 is produced by the avalanche event and is directed within thesubstrate 12 toward the back surface 51. A first radiated photon 66reflects off of the back surface 51 at the location of an aperture 62 inthe layer 60. This photon is reflected back into the substrate 12 andcan be directed to a neighboring photodiode 16, causing a secondaryavalanche event at the neighboring photodiode 16. In contrast, radiatedphotons 68 and 70 reach the back surface layer 60 at a location wherethe layer material is continuous. These photons 68, 70 may be absorbedby the layer 60, in which case they cannot cause secondary avalancheevents at neighboring photodiodes 116, 18. In FIG. 6, the dotted opticalrays are intended to indicate attenuation of rays.

With this secondary photon absorption technique, it is demonstrated inFIG. 6 that it can be preferred to conduct optical ray tracing analysesto determine an appropriate aperture geometry and aperture pattern forthe material layer 60 that both enables incoming photons to reachphotodiodes and that absorbs photons that are radiated from a photodiodeduring a primary avalanche event.

Example 1

An APD array substrate of InP was fabricated with mesa photodiodes ofthe SAM configuration and with no back side surface cross-talk blockinglayer. A second InP APD array was similarly produced but was providedwith a back-surface cross-talk blocking layer having 40 micron-diametercircular openings, each opening located above an underlying photodiodemesa, with a resulting 50 micron-pitch. The back surface cross-talkblocking layer consisted of a blanket coating of titanium of 50 nm inthickness and a blanket coating of gold of 250 nm in thickness, with thecircular openings produced by lift-off photolithography on the titaniumand gold layers.

The optical cross-talk between neighboring photodiodes in a 9×9 area wasdetermined for each of the APD arrays as a function of applied over biasto each photodiode for arming the photodiodes above breakdown to detectincoming photons. FIG. 7 is a plot of an integrated cross-talkprobability corresponding to this determination. Specifically, this plotpresents the probability of more than one cross-talk avalanche eventoccurring, integrated over 20 ns after a primary avalanche event, forany primary avalanche events occurring in a 9×9 area of photodiodes.

The plot demonstrates that for a given over bias, e.g., 2 V, the backsurface TiAu cross-talk blocking layer provided three orders ofmagnitude reduction in more than one cross-talk avalanche event for the9×9 neighborhood of photodiodes. This surprisingly superior performancedemonstrates that with appropriate design of the cross-talk blockinglayer pattern, significant reduction in cross-talk can be achieved.

FIGS. 8A-8B present additional probability data for the TiAu-coated APDarray substrate and the bare APD array substrate. In FIG. 8A there isplotted for the bare APD array substrate the integrated cross-talkprobability, P, for the 20 ns integration period and across the 9×9photodiode array, that ≧1 secondary avalanche event occurs, and that ≧2secondary avalanche events occur, with these probabilities also plottedfor the inherent background noise of the APD.

FIG. 8B presents the same four data plots of secondary avalanche eventprobability for the APD array substrate including the TiAu patternedcross-talk blocking layer. These plots demonstrate the dramaticreduction in cross-talk that is achieved by the TiAu cross-talk blockinglayer.

Now turning to further details in the design and selection of thecross-talk blocking layer, as explained above and demonstrated in theabove example, more than one material can be included in the layer. Forexample, it is found that some combinations of materials can provide alayer that is characterized by a quite strong evanescent mode forcapturing a photon directed to the back surface of the APD arraysubstrate.

FIG. 9 provides plots of the percentage of internal reflection for 1030nm photons striking the back surface of the APD array substrate as afunction of the incident angle at which photons strike the surface fortwo different combinations of cross-talk blocking layer materials. Afirst example combination of materials examined here is TiAu, as wasdescribed in the example above. A second combination of materialsexamined here is GeCr. Both s-polarized and p-polarized light is hereconsidered for the two material combinations.

From the plot it is seen that the GeCr cross-talk blocking materiallayer combination provides a significant reduction over the TiAucombination in broad spectrum reflection from the back surface of theAPD array substrate. It is found that the GeCr layer stack ischaracterized by a strong evanescent mode that effectively capturesradiation directed to the stack. This analysis is an example of designconsiderations that can be made to optimize the selection of materialsto be included in the cross-talk blocking layer depending on thesubstrate dielectric properties, primary operating wavelength, andspectrum of avalanche emission that are characteristic for a given APDarray.

Referring also to the plot of FIG. 10, the thicknesses of the materialsselected to be included in the cross-talk blocking layer further can beoptimized to maximize suppression of optical cross-talk. FIG. 10provides plots of the percentage of optical reflection from the backsurface of the APD array substrate for the GeCr material layercombination, for a 50 nm-thick Ge layer and for different Cr layerthicknesses, namely, 9 nm, 15 nm, 22 nm, and 25 nm, as a function of thewavelength of the secondary emission spectra. This data represents ananalysis to determine the optimum Cr thickness, when paired with a Gelayer, for maximizing absorption of secondary photons in the APD arraysubstrate.

As shown in the plot, for this analysis it is found that an increase inCr material thickness produces a reduction in optical reflection fromthe GeCr surface layer, with a reflection minimum at a wavelength ofabout 1000 nm. This result is particularly important because asexplained below, significant avalanche spectral emission occurs nearthis 900-1100 nm range of wavelengths, and a minimum in reflection fromthe GeCr surface layer is achieved at this range. This data analysistherefore demonstrates a further design consideration of materialthickness for minimizing reflection of secondary avalanche emissionsfrom the back surface of the APD array substrate for the spectralemission expected for a given APD array. Taken together, cross-talkblocking layer material composition and thickness, along with aperturegeometry and pattern, all based on APD array geometry and photonwavelengths of interest, enable a cross-talk blocking layer thatsignificantly suppresses optical cross-talk due to secondary emissionfrom avalanche events in the APD array.

As explained above, any in a wide range of cross-talk blocking layerarrangements can be employed. In a further example, a continuous blanketof cross-talk blocking layer material, rather than a layer withpatterned apertures, can be provided on the back surface of the APDsubstrate. The blanket layer of material is characterized as allowing areasonable transmission of primary photons through the layer to reachphotodiodes in the array while still acting to attenuateavalanche-emitted light incident at larger off-normal angles. Thematerial examples described above can here be employed where suitable.Evanescent mode coupling to a thin Ge layer is one example of operationof a continuous blanket-coated cross-talk blocking layer that can beprovided on the back surface of the APD substrate.

Other configurations can be employed to include spatial elements, i.e.,physical structural elements, in an APD array and/or photodiodestructure to manipulate the optical path between photodiodes in thearray. For example, patterned blocking layers can be located within thebulk of the APD array substrate at specific locations that suppressreflected cross-talk emission.

FIGS. 11A-11B are schematic sectional side views of example APD arraysubstrate configurations including such patterned blocking layers. In afirst example configuration of an APD detector focal plane array 80including an APD substrate 12 with blocking layers, the substrateincludes bulk regions 84, 86, 88, of a selected substrate material,e.g., InP. Two or more of these bulk regions are separated by a plane inwhich there is disposed a patterned cross-talk blocking layer, e.g.,layers 90, 92, of a selected material that is compatible with the bulkregions. For example, given InP bulk regions, a blocking layer of InGaAscan be suitable. The back surface of the APD array substrate can alsoinclude the cross-talk blocking material layer 60 described above, aswell as an antireflective coating 53. The microlens array 30 here isshown separated from the substrate 12 by an air gap 31, withantireflective coatings 53 provided on the microlens surfaces.

The bulk blocking layers 90, 92, are provided with a selected aperturepattern, e.g., the pattern of circular apertures 62 shown in FIG. 5. Theblocking layers can include identical or distinct aperture patterns.Each blocking layer 90, 92, includes continuous regions of blockingmaterial having a composition and geometry that are selected to absorbavalanche photons that are traveling through the substrate. Inoperation, secondary photons 91 traveling through the substrate from aphotodiode that strike one or more such blocking regions may be absorbedby the blocking regions, reducing the likelihood that such photons couldcause a secondary avalanche event at a photodiode neighboring thelocation of a primary avalanche event. Any number of blocking layers andblocking regions within a given blocking layer can be included forreducing optical cross-talk within an APD array substrate, givenconsideration of material and processing constraints.

The bulk blocking layers thereby operate in the manner of the cross-talkblocking layer 60 on the back surface, with apertures for enablingincoming radiation to reach photodiodes 14, 16, and continuous regionsfor attenuating radiation that is produced by avalanche events.Radiation 91 directed from a photodiode 14 during an avalanche event maybe attenuated at one or more of the bulk blocking layers 90, 92, and/orat the back surface blocking layer 60. The dotted rays are intended inthe figure to represent attenuated rays. The bulk blocking layersthereby provide additional attenuation of avalanche radiation andcorresponding reduction in optical cross-talk.

As shown in FIG. 11B, such a configuration can be modified further toprovide a surface 102 of the substrate 12 that can be bonded to amicrolens array 30, in a configuration that provides optimal substratethickness and blocking array locations to minimize cross-talk within theAPD array substrate. One or more layers of material can be includedbetween the substrate and the microlens array to achieve a selecteddielectric arrangement.

With the arrangement shown in the figure, secondary photons 91 producedby an avalanche event that travel through the substrate 12 may beabsorbed at blocking regions in the blocking layers 90, 92, as in thearrangement of FIG. 11A, either as they travel directly from aphotodiode or after reflecting off of the top surface of the microlensarray 30. Radiation can thereby be attenuated more than once as thatradiation travels through the substrate. The dotted rays in the figureare intended to indicate attenuation of those rays.

This configuration of substrate-to-microlens bonding is effective forcutting off light reflected from the areas of the back side of the focalplane array that may not be available for sufficient back sidecross-talk blocking layer placement due to placement of antireflectioncoatings for primary photon coupling or other required elements.

Turning now to the design specifics of each photodiode in the APD array,referring to FIG. 12 there are shown two adjacent photodiodes 14, 16,disposed on the APD array substrate 12 in the manner described above.For clarity of discussion, the APD array substrate is shown hereinverted from the view in the figures above, e.g., FIG. 3. The APD arraysubstrate is shown here including the back side cross-talk blockinglayer 60 described above for attenuating radiation that is directed tothe back surface 51 during an avalanche event.

Each photodiode is provided as a structure of layers, here as aninverted mesa geometry having sidewalls that are preferably sloped tosuppress edge breakdown at high-field regions of the mesa. In oneexample configuration of photodiode layers shown in the example of FIG.12, there is provided an arrangement of n-type layers on p-type layers,but such is not required by the invention; any suitable arrangement ofdoped layers can be employed. At the top of each mesa there is provideda bump contact 42, made to a top contact 150 of each photodiode 14, 16,e.g., with indium, for bump bonding to the ROIC circuitry in the mannerdescribed above. In the n-type-on-p-type example configuration here, ann⁺ contact 152, e.g., InGaAs, is provided under the top contact 150.

The photodiode absorber layer 154, as an example here an n⁻ InGaAsPabsorber layer, is provided below the contact layer 152 separated by anepitaxial layer 156, e.g., of n⁺ InP. The photodiode avalanche layer, ormultiplier layer 158, as an example here an n⁻ InP multiplier layer, isprovided below the absorber layer 154, separated by a field stop layer160, here as an example an n⁺ InP layer. The lower contact 162, i.e.,anode, of the photodiode, here as an example a p+ InP layer, is providedbelow the multiplier layer 158. A passivating material 164, e.g., alayer of polyimide or other suitable material, is provided over each ofthe photodiode mesas, covering the mesa sidewalls and the lower anode ofeach photodiode.

In accordance with the invention, there can be incorporated in the APDarray a spectral filter layer designed to pass the wavelengths ofinterest, i.e., to allow wavelengths to be detected by the APD array toreach the photodiodes, while attenuating other regions of the broad hotcarrier avalanche emission spectrum of secondary photons that areproduced in the APD array. In one example, a doped semiconductor layeris provided as the spectral filter layer, designed with a selectedcomposition that is characterized by a band gap that absorbs theunwanted regions of the radiation spectrum. The APD structure canincorporate both this spectral filter layer and the cross-talk blockinglayer described above to achieve maximum reduction in optical cross-talkin the APD array.

Referring to FIG. 12, such a spectral filter layer 166 that isincorporated into the APD array configuration for attenuating secondaryradiation in the APD array can in one example, shown here, be providedunder the lower photodiode contact 162. In this example, the spectralfilter layer 166 is provided as a p⁺ InGaAsP layer, and extends underall of the mesa photodiodes. In other words, the spectral filter layer166 is a continuous blanket layer on the APD substrate, with thephotodiode mesas disposed on the continuous spectral filter layer. Suchis not in general required by the invention, however. The spectralfilter layer can be discontinuous across the array or continuous,as-suitable for a given application and APD array geometry.

The bandgap of each of the photodiode absorber and multiplier layers andspectral filter layer of the APD array are specified in concert toachieve efficient and effective Geiger-mode avalanche operation for aselected wavelength or range of wavelengths to be detected as well as tominimize optical cross-talk during photodiode avalanche events. Ingeneral, each of the layers of the photodiodes in the APD arraysubstrate are formed of selected semiconducting materials that providean appropriate band gap for the function of each layer. The absorberlayer material is characterized by a band gap that corresponds to awavelength of interest to be detected by the photodetector. Photonsabsorbed by the absorber layer generate corresponding electronic chargecarriers, and these charge carriers are multiplied by an avalanche eventin the multiplier layer for triggering the APD detection circuitry. Withthis operation, semiconductor materials such as Si, Ge, GaAs, InP, GaSb,InGaAs, InGaAsP, CdTe, ZnS, and other such materials are particularlysuitable as APD layers.

The cut-off wavelength of an APD, i.e., the longest wavelength ofincoming photons that can be detected by the APD, is determined by thebandgap energy of the photodiode layer having the smallest bandgap. Atwavelengths longer than this smallest bandgap, incoming light is notstrongly absorbed by the photodiode absorber layer. For example, InGaAslayers can be employed to absorb photons having a wavelength that isless than 1.6 μm, and combined with InP or InAlAs layers to multiply thephoto-generated carriers produced in the InGaAs.

During an avalanche event, as photo-generated electronic charge carriersare multiplied, some recombination and relaxation of carriers andemission of photons occurs, as explained above, and such emission canresult in optical cross-talk across the APD array. Avalanchingphotodiodes tend to emit photons over a broad range of wavelengths, bothabove and below the bandgap of the avalanche multiplier layer material.FIG. 13 is a plot of the measured spectrum of avalanche emission ofphotons during an avalanche event, from an InP multiplier layer likethat in the photodiode structure of FIG. 12. This spectrum shows a peakin number of emitted photons near the InP bandgap wavelength of 910 nm.The spectrum further is characterized by a broad thermal component withan equivalent temperature near about 3000 K.

For photons generated by the multiplier layer that have a wavelengthless than the InP bandgap of 910 nm, the photons are reabsorbed in InPlayers of the photodiode or in the nearby substrate, and therefore arenot transmitted into the APD array substrate. For photons generated bythe multiplier layer that have a wavelength longer than the InGaAsbandgap, the photons are emitted into the APD array substrate, butcannot be strongly absorbed by the absorber layer of other photodiodes.But any photons generated by the multiplier layer having intermediatewavelengths between the bandgap wavelength of the multiplier layermaterial and the bandgap wavelength of the absorber layer material, inthis example between 910 nm and 1600 nm, can be emitted from anavalanching photodiode into the APD array substrate and absorbed by aneighboring photodiode in the array, causing the neighboring photodiodeto initiate an avalanche event. That is, any photons generated by theavalanche event with a wavelength shorter than about 1600 nm couldtrigger another correlated avalanche event in the APD array due to crosstalk in InGaAs/InAlAs or InGaAs/InP APDs.

The spectral filter layer 166 of the photodiode array, shown in FIG. 12,is designed to filter out of the APD substrate such photons generatedfrom an avalanche event that could be reabsorbed into neighboringphotodiodes, while allowing photons in the primary detection band, i.e.,the pass band of radiation wavelengths to be detected by the photodiodearray, to reach the photodiodes in the array. This is particular usefulfor applications in which an APD array is to be employed for detecting aspecific wavelength, such as in active laser radar or lasercommunication receivers.

For example, corresponding to the example material combination shown inFIG. 12, there is presented in FIG. 14 the spectral transmissioncharacteristics for an InGaAs APD absorber layer and two differentspectral filter layers, one bulk, one a superlattice, designed foroperation near 1550 nm. The In.60 Ga.40 As.85 P.15 bulk spectral filterlayer 166 considered in FIG. 14 will tend to absorb avalanche-emittedphotons shorter than 1550 nm, while allowing low-loss transmission ofprimary photons, i.e., photons 38 of 1550 nm in wavelength, to theInGaAs APD absorber layer 154.

A similar filter characteristic can be achieved using multiple thinnerlayers of high and low bandgap energy materials, e.g., configured as asuperlattice. The spectral filter cutoff characteristic of one exampleof such a superlattice filter is also shown in FIG. 14 for comparisonwith the bulk In.60 Ga.40 As.85 P.15 filter. The superlattice in thisexample utilizes 200 periods of alternating 56 angstrom-thick InGaAslayers and 60 angstrom-thick InP layers to form a superlattice with aneffective cutoff wavelength nearly equivalent to a bulk InGaAsP filterlayer.

In another example, given the InGaAsP absorber layer 154 and InPmultiplier layer 158 in the example photodiode of FIG. 12, thephotodiode configuration can be designed to operate at about 1064 nm.The InGaAsP spectral filter layer 166 is here provided with a bandgapthat is adjusted for absorbing slightly shorter wavelengths than 1064nm; i.e., the spectral filter layer is characterized by a bandgap thatis between that of the InP multiplier layer and the InGaAsP absorberlayer. Thus, as seen by this example, the spectral filter layer isspecifically designed to pass a range of wavelengths of interest to bedetected by the APD array while attenuating other regions of the broadhot carrier avalanche emission spectrum. The composition of the spectralfilter layer is therefore preferably a semiconductor having a bandgapthat absorbs the unwanted shorter-wavelength regions of the avalancheradiation spectrum.

The APD absorption layer is therefore designed in concert with thespectral filter layer to provide the absorption/filter combinationdesired for a given application. Preferably, the APD absorption layer isdesigned with the spectral filter layer to maintain reasonablelattice-matching to the substrate, e.g., the InP substrate in theexample of FIG. 12, and at the same time to give appropriate band gapsto eliminate, or significantly reduce, sensitivity of the APD array towavelengths other than the primary source. This can be achieved bytailoring the composition of the APD absorber layer to maximizeabsorption close to the primary wavelength of interest, e.g., 1064 nmfor the InGaAsP/InP example, while making sure that this layer doesn'tabsorb light of wavelength much longer than that of the primary source,i.e., >1064 nm. The spectral filter layer of InGaAsP is then fine tunedto attenuate light of wavelengths shorter than the primary source, e.g.,1064 nm. For example, given a range of various GaInAsP alloys,transmission of a 1030 nm source can be maximized while significantlyattenuating shorter wavelengths by selection of a synergisticcombination of absorber layer and filter layer materials.

Accordingly, in practice, it can be preferred to set the absorber andspectral filter layer bandgaps so that a wavelength spectrum of, e.g.,between about 20 and 200 nm width is provided for transmission, ratherthan attenuation, of incoming radiation. FIG. 15 is a plot of an exampleof an optimization analysis of the cutoff wavelength of the absorberlayer to reduce the spectrum of avalanche cross-talk and retain highphoton detection efficiency for the wavelength of interest, here 1060nm. In practice, the absorber cut-off wavelength is shifted until thedetection efficiency begins to drop at the temperature of operation. Asis shown in the plot, some incoming primary photons may be lost to thespectral filter layer, and thus it can be preferred to fine tune thecutoff wavelength of the spectral filter layer.

Because many semiconductor bandgap energies shift with temperature, aknown fixed operating temperature allows for the design of a spectralfilter layer and photodiode absorber layer pair with a narrower responseband. For example, FIG. 15 shows the ˜14 nm shift in the absorber layercutoff wavelength as the operating temperature changes from 300K to260K. A similar temperature change would induce ˜30 nm shift in thecutoff wavelength in the InGaAs APD absorber designed for 1550 nm, as inFIG. 14. Thus the optimum APD array design including a spectral filterlayer for a variable operating temperature preferably is characterizedby a spectral filter cut-off designed for warmest operating temperatureand APD absorber cut-off designed for coolest operating temperature,resulting in a wider separation between the filter and absorber.

The plot here assumes that avalanche cross-talk photons make a“double-pass” through the spectral filter layer. Referring back to FIG.12, in considering this “double-pass” of cross-talk photons through thespectral filter layer, two neighboring photodiodes are shown, and thefunction of the spectral filter layer 166, in combination with the backsurface cross-talk blocking layer 60, for significantly reducingcross-talk in the APD array between the neighboring photodiodes isdemonstrated. Primary incoming photons 38 are incident at the APD arrayat the optical input surface 51 of the array. The photons are directedthrough apertures in the back surface blocking layer 60 to photodiodes.Given absorption of an incoming primary photon at a first photodiode 14,a secondary avalanche photon 175 is generated in the multiplier layer158 of the photodiode 14 during an avalanche event that was initiated byabsorption of the primary incoming photon.

The secondary photon 175 is directed to the APD array substrate andtraverses the InGaAsP spectral filter layer 166, at which the photon canbe absorbed, thereby significantly attenuating the avalanche radiationin the APD array substrate. The reduced radiation 175 then reaches theabsorber material of the back surface layer 60 where the radiation isfurther, if not completely attenuated. Reflecting off of the backsurface layer 60, the photon 175 then is directed back to the APD arraysubstrate, again traversing the InGaAsP spectral filter layer 166. Thissecond traversal, or double-pass, of the spectral filter layersignificantly, if not completely attenuates the avalanche radiation. Fora large population of such secondary photons, this second traversal ofthe spectral filter layer absorbs the photon. Any secondary photons notabsorbed by the spectral filter layer that can be absorbed at theabsorber layer 154 of a neighboring photodiode can cause an avalancheevent at that neighboring photodiode. Thus, the continuous spectralfilter layer enables the double pass of an avalanche photon on itstraversal from a first photodiode through the APD array substrate to theback surface of the substrate and on to a neighboring photodiode. Withthis arrangement the spectral filter layer provides a particularlyeffective filter configuration in an elegantly simple geometry.

The back-surface cross-talk blocking layer 60, when incorporated intothe APD array with the spectral filter layer 166 as in the example ofFIG. 12, is preferably designed based on the requirements for thespectral filter. For example, the cross-talk blocking layer can bedesigned to absorb a narrow band of wavelengths that might by necessitybe transmitted through the spectral filter layer and could be absorbedby another APD. Thus, the characteristic absorption of the cross-talkblocking layer materials, when included with a spectral filter layer inthe APD array, needs to be achieved only over a relatively narrow bandof wavelengths that are not addressed by the spectral filter layer. If aspectral filter layer is not included in the APD array, then thecross-talk blocking layer is required to attenuate back-side radiationreflection over a larger range of wavelengths. Accordingly, it can befound for many applications that a combination of the spectral filterlayer and the cross-talk blocking layer can be preferred to ease designconstraints.

Example 2

An APD array including photodiodes having the configuration illustratedin FIG. 12 was fabricated with the semiconductor alloy layers specifiedin FIG. 12. The thicknesses and compositions of the layers were asfollows: 1.5 μm-thick In. 89 Ga.11 As.23 P.77 absorber layer, having a1055 nm bandgap, 1.4 μm-thick InP multiplier layer, 1 μm-thick In.92Ga.08 As.16 P.84 spectral filter layer, having a 1011 nm bandgap. Asimilar APD array was fabricated, but not including the spectral filterlayer. The thicknesses and compositions of the layers were as follows:1.5 μm-thick In.81 Ga.19 As.42 P.58 absorber layer, having a 1175 nmbandgap, and a 1.4 μm-thick InP multiplier layer. Both APD arraysincluded a back-side cross-talk blocking layer. For the array includinga spectral filter layer a 50 nm-thick Ge layer and a 25 nm-thick Crlayer were deposited on the back surface. For the APD array without aspectral filter layer the back surface layer consisted of a coating oftitanium of 50 nm in thickness and a coating of gold of 250 nm inthickness. Both arrays included HfO₂ anti-reflection coatings insidecircular apertures that were provided in the backside blocking layers.

FIG. 16 is a plot of the relative quantum efficiency as a function ofwavelength for the two APD arrays. The plot with closed circlescorresponds to the APD array including the spectral filter, and thecurve with star points corresponds to the APD array not including aspectral filter. The short wavelength response is limited by the InPsubstrate for the array without the spectral filter, whereas the cut-offis shifted to longer wavelengths in the APD array with the spectralfilter. The spectral filter layer absorbed photons having wavelengthscorresponding to the bandgap of the filter layer (1011 nm) and shorter.This limits sensitivity of the APD array to 920-1030 nm photons thatmight be emitted in an avalanche event. Comparing the long wavelengthcut-off of the two arrays, the bandgap of the absorber has been shifteddown to 1055 nm in the array including the spectral filter. This APDarray retains high efficiency at 1030-1050 nm, while reducingsensitivity to 1060-1200 nm photons that might be emitted in anavalanche event.

FIG. 17 is a plot of the spectral response for the two APD arrays as afunction of wavelength. Also plotted is the avalanche photon emissionspectrum for the InP multiplier layer. The plot with closed circlescorresponds to the APD array including the spectral filter, and thecurve with star points corresponds to the APD array not including aspectral filter. The spectrum of cross-talk to avalanche emission forthe APD array including a spectral filter layer is found to be limitedand centered at around 1040 nm. The filter rejects the majority of thelarge peak in avalanche emission at shorter wavelengths near 950 nm. Thedata thereby demonstrates that the avalanche emission spectrum is verysignificantly attenuated by the spectral filter layer. Additionally,this shows the benefit of including an APD absorber layer that is tunedclosely to the primary photon wavelength of interest.

FIG. 18 is a plot of the integrated probability of causing two or morecross-talk events for a 9×9 area of photodiodes, over a duration of 20ns after an avalanche event, as a function of over bias voltage ofphotodiodes for several different spectral filter layer designs. All ofthe APDs here are 20 microns in diameter, all APD absorber layers are1.5 microns-thick, and all spectral filter layers are 1 micron-thick inthe designs that include spectral filters. Shown in the curve with starpoints in the plot is the cross-talk probability for an APD arraywithout a spectral filter layer, but including a 50 nm-thick Ti and 200nm-thick Au back side cross-talk blocking layer and an In.81 Ga.19 As.42P.58 APD absorber layer, characterized by a 1175 nm bandgap.

Also shown is the cross-talk probability for APD arrays including fourdifferent combinations of spectral filter layers and APD absorberlayers. The curve marked by open squares corresponds to an APD arrayincluding a 1.5 μm-thick In.81 Ga.19 As.42 P.58 absorber layer of 1175nm in bandgap and 1 μm-thick In.91 Ga.09 As.19 P.81 spectral filterlayer of 1030 nm bandgap, and also includes a 50 nm-thick Ti and 200nm-thick Au back side cross-talk blocking layer. The solid diamond curvecorresponds to an APD array including a 1.5 μ-m-thick In.82 Ga.18 As.38P.62 absorber layer of 1150 nm in bandgap and a 1 μm-thick In.92 Ga.08As.18 P.82 spectral filter layer of 1020 nm in bandgap, with a 50nm-thick Ti and 200 nm-thick Au back side cross-talk blocking layer. Thesolid triangle curve corresponds to a 1.5 μm-thick In.86 Ga.14 As.29P.71 absorber layer of 1090 nm in bandgap and a 1 μm-thick In.92 Ga.08As.16 P.84 spectral filter layer of 1013 nm in bandgap, with a 50nm-thick Ge and 25 nm-thick Cr back side cross-talk blocking layer. Thesolid circle curve corresponds to an APD array including a 1.5 μm-thickIn.89 Ga.11 As.23 P.77 absorber layer of 1055 nm in bandgap and a 1μm-thick In.92 Ga.08 As.16 P.84 spectral filter layer of 1013 nm inbandgap, with a 50 nm-thick Ge and a 25 nm-thick Cr back side cross-talkblocking layer.

As can be seen by the results in FIG. 18, the integrated cross-talk isreduced by roughly a factor of 100 compared to the array without aspectral filter layer. Cross-talk is reduced in each successive curve asthe spectral bandwidth between the spectral filter cut-off and APDabsorber cut-off is reduced.

Turning now to considerations for fabrication of the APD array, noparticular fabrication sequence is required by the invention. Any in awide range of semiconductor materials processing techniques can beemployed for producing an APD array. In one example process forproducing an APD array like that of FIG. 12, the APD structures aregrown by organometallic vapor phase epitaxy (OMVPE) on (1 0 0) InPsubstrates. All of p⁺, n⁺, and n⁻ substrate types can be employed. Thespectral filter layer 166 of p⁺ InGaAsP of 1 micron in thickness isgrown first, and then a two-step p⁺ InP layer 162 growth is carried outto produce the anode. The lower part of this anode layer is about 1.5 μmthick and includes Zn doping of 1.6×10¹⁸ cm⁻³, while the upper part isabout 0.5 μm-thick and is doped at about 8×10¹⁷ cm⁻³. The nominallyundoped InP avalanche layer 158 is grown next, having a thickness ofabout 0.8-2.0 μm, with an n-type concentration of about 10¹⁵ cm⁻³. Thisis followed by a heavily Si-doped n⁺ InP field stop layer, of 3.5−7×10¹⁷cm⁻³. The thickness of this layer is selected, based on the layerdoping, so that first, the absorber layer is fully depleted at thephotodiode over bias and temperature of operation and second, themaximum field in the absorber layer is below a maximum value at theoperating conditions. For an APD array designed for incoming radiationof 1.06 μm wavelength, the maximum field in the absorber is kept belowabout 1×10⁵ V/cm, while for 1.55 μm wavelength APD arrays, it is kept assmall as possible, e.g., about 10⁴ V/cm. The first criterion assuresthat photo-carriers generated anywhere in the absorber layer are sweptquickly to the avalanche layer, thereby reducing jitter. The secondcriterion minimizes any field-enhanced dark current in the absorberlayer.

A compositionally-graded InGaAsP layer can be included, if desired, tofacilitate the injection of photogenerated holes from the absorber layerinto the avalanche layer. This layer is not shown explicitly in thefigures as it is of the same materials as the absorber layer, but it isto be understood that such is included in the photodiode structure. Thegraded layer can be, e.g., about 50 nm in thickness for a 1.06 μmwavelength photodiode and about 100 nm in thickness for a 1.55 μmwavelength photodiode. The nominally undoped InGaAsP or InGaAs absorberlayer, having a doping level of less than about 10¹⁵ cm⁻³, of thicknessabout 1.5 μm is then grown. This is followed by an n⁺ InP layer and a 10nm-thick n⁺ contact layer. For all layer growth steps, a growthtemperature of about 625° C. can be preferred to minimize dark countrate.

With the photodiode layers grown, the photodiode mesa structures can beproduced if such are desired for a given application. The mesafabrication process preferably isolates individual photodiodes withoutintroducing defects or current paths that could degrade photodiodeperformance. In one mesa fabrication process, the grown active layersare etched completely through to the spectral filter layer 166. The mesaetch can be conducted as, e.g., either a nonselective wet etch or aninductively-coupled plasma reactive ion etch followed by a brief wetcleanup etch to remove ion damage. Large APD array fabrication generallyrequires the use of such a dry etch process because the dry etch processproduces a more spatially-uniform etch across a wafer than a deep wetetch. After mesa etching, passivation is applied and a cathode ohmiccontact is made to the top of the mesas. If the photodiode array isconfigured as in FIG. 12, as back-illuminated devices mated to ROICs,then a disk-shaped contact is used, while for top-illuminated devices,an annulus-shaped contact is used. Anode contacts are either made to theback of the APD array substrate or on the top to the etched p⁺ anodelayer.

For a back-illumination configuration, the substrate is thinned to about150 μm and antireflection coatings applied to the back surface. Afterphotolithography and etching to define the antireflection-coatedapertures, the backside cross-talk blocking layers are evaporated. Inone example process, 50 nm of Ge is evaporated, followed by evaporationof 25 nm of Cr, on the entire back surface. A lift-off procedure is thencarried out to remove the backside material from the regions above theremaining antireflection-coated apertures, producing a self-alignedduality of antireflection coating apertures and backside blocking layermaterial regions.

Passivation materials such as polyimide, polyimide overcoated withsilicon nitride or silicon dioxide, bisbenzocyclotene (BCB),hydrogensilsesquioxanes (HSQ), pyrolytic silicon dioxide, regrown InP,or other suitable material can be employed for passivation of thephotodiodes in the APD array. A polyimide-silicon nitride two-layercoating can be preferred because polyimide coats each mesa andpassivates the semiconductor surface and an overcoating of siliconnitride fills in any microcracking and protects the polyimide from lowlevels of moisture.

A microlens array for mating with the APD array can be provided as,e.g., GaP or other suitable material, with a lens-to-lens pitch of,e.g., about 50 μm and a thickness of, e.g., about 100 μm. The microlensarray is preferably coated with antireflection coatings on all surfacesand paired with an APD array substrate of appropriate thickness for thefocal length of the lens; the substrate can be thinned to match themicrolens focal length. Preferably the lens array and APD array areprecisely aligned, e.g., with active alignment diodes that are providedat, e.g., corners of the APD array, that produce photocurrent which canbe monitored as the arrays are actively aligned. Bump bonding, e.g.,with In bumps, can be completed between the APD array and a ROIC priorto microlens array attachment.

This example fabrication sequence can be modified as-necessary toaccommodate various features in the APD array substrate. For example,bulk cross-talk blocking layers can be incorporated into the APD arraysubstrate as in the configurations illustrated in FIGS. 11A-11B. FIGS.19A-H are schematic cross-sectional side views of the APD platformduring fabrication of such bulk blocking layers.

As shown in FIG. 19A, on a selected substrate, e.g., a (0 0 1) InPsubstrate, there is grown by OMVPE an InP buffer layer 200, a GaInAsetch stop layer 205, an InP spacer layer 210, and a GaInAs bulkcross-talk blocking layer 212. This blocking layer 212 is patterned, asshown in FIG. 19B, to include continuous regions 214 and apertures 216,as described above. The apertures are provided to enable incoming lightto reach photodiodes supported by the substrate 12 while the continuousregions 214 are provided to attenuate avalanche photon emission in themanner described above.

Referring to FIG. 19C, InP is then regrown, e.g., by HVPE, in theaperture regions 216 of the blocking layer 212 and further is grown ontop of the blocking layer to form an InP layer 220. As shown in FIG.19D, a second blocking layer 222 can then be grown, e.g., by OMVPE, andas shown in FIG. 19E, patterned to form apertures 225 in the continuousblocking layer 222. Then as shown in FIG. 19F, HVPE InP growth iscarried out to fill the apertures 225 with material and to form anadditional InP layer 230. This process of stacking blocking layers onInP spacer layers can be continued as-desired to form any selectednumber of bulk blocking layer structures.

As shown in FIG. 19G, OMVPE growth of the APD array photodiode layers235, including a spectral filter layer, is then conducted on the top InPregrowth layer 230. The InP substrate 12 can then be removed, e.g., byan etch that stops on the GaInAs etch stop layer 205, and the microlensarray, e.g., the GaP array described above, can be bonded directly tothe bottom InP spacer layer 210 as shown in FIG. 19H. With this step,the substrate-to-lens bonding structure of FIG. 11B is complete.

Other fabrication processes can be employed in accordance with theinvention to produce any suitable photodiode array structure and focalplane stack. The invention is not limited to a particular fabricationsequence and is not limited to a particular set of photodiode materials.Any materials that enable production of G-M avalanche photodiodeoperation can be employed.

With this discussion, there is provided both the fabrication sequenceand design of an APD array with significantly reduced opticalcross-talk. As explained above, cross-talk in Geiger-mode APDs canresult in a range of operational limitations, depending on theapplication of the APDs. Cross-talk can cause ghosting/blurring ofangle-angle-range images in 3-D laser radar applications, and can inducebit errors in single-photon communication application. Because G-M APDsmust be held in an “off” state for a period of time prior to detectionof succeeding photons, APDs that are triggered due to cross-talk cannotdetect another photon until they are reset. This can cause prematuresaturation of an APD receiver, perhaps even blinding an APD array beforeprimary photons are detected. Because cross-talk is a super-linearprocess, reduction in the strength of cross-talk by a factor of 2-8, asprovided by the invention and described above, can eliminate such arunaway process that could otherwise blind the majority of APDs in anarray.

The spatial blocking layers and spectral filters described above forreducing APD cross-talk are easily fabricated, requiring only thealready existing processes utilized for G-M APD array fabrication. Thespectral filter layer can be grown along with the APD array structure,with no additional fabrication steps required. Optimization of thefilter layer material, in concert with optimization of the absorberlayer material, for specific cut-off wavelengths and a selected opticalspectrum pass band requires only modest recalibration of growth systemparameters prior to APD array growth. The spatial blocking layers andthe spectral filter can be implemented in silicon, in silicon-basedmaterial systems, in III-V material systems, other II-VI systems, orother suitable material systems, and thus these elements are not limitedto a particular material system.

It is recognized, of course, that those skilled in the art may makevarious modifications and additions to the embodiments described abovewithout departing from the spirit and scope of the present contributionto the art. Accordingly, it is to be understood that the protectionsought to be afforded hereby should be deemed to extend to the subjectmatter claims and all equivalents thereof fairly within the scope of theinvention.

1. An avalanche photodiode detector comprising: a substrate including anarray of avalanche photodiodes; an optical interface surface of thesubstrate arranged for accepting external input radiation; and at leastone cross-talk blocking layer of material including apertures positionedto allow external input radiation to reach photodiodes and includingmaterial regions positioned for attenuating radiation in the substratethat is produced by photodiodes in the array.
 2. The avalanchephotodiode detector of claim 1 further comprising a microlens arraydisposed at a position relative to the substrate to focus external inputradiation to the photodiodes.
 3. The avalanche photodiode detector ofclaim 2 wherein the microlens array is adjacent to and separated fromthe substrate.
 4. The avalanche photodiode detector of claim 2 whereinthe microlens array is disposed on the optical interface surface of thesubstrate.
 5. The avalanche photodiode detector of claim 1 furthercomprising connections from the substrate to photodiode control andread-out circuitry.
 6. The avalanche photodiode detector of claim 1wherein the radiation produced by a photodiode comprises photons emittedby the photodiode during a Geiger-mode avalanche event at thatphotodiode.
 7. The avalanche photodiode detector of claim 1 wherein thecross-talk blocking layer apertures comprise an array of circularapertures.
 8. The avalanche photodiode detector of claim 1 wherein theat least one cross-talk blocking layer comprises at least one cross-talkblocking surface layer disposed on the optical interface surface of thesubstrate.
 9. The avalanche photodiode detector of claim 8 wherein thecross-talk blocking surface layer comprises a material for suppressinginternal reflection of photons, produced by photodiodes, at the opticalinterface surface.
 10. The avalanche photodiode detector of claim 1wherein each photodiode comprises a mesa structure of photodiode layersand the apertures in the at least one cross-talk blocking layer allowinput radiation to reach the photodiode mesas.
 11. The avalanchephotodiode detector of claim 10 wherein the photodiode mesas aredisposed at a substrate surface opposite the optical interface surfaceof the substrate.
 12. The avalanche photodiode detector of claim 1wherein the substrate comprises a semiconducting substrate.
 13. Theavalanche photodiode detector of claim 12 wherein the substratecomprises an InP substrate.
 14. The avalanche photodiode detector ofclaim 1 wherein the at least one cross-talk blocking layer comprises ametal.
 15. The avalanche photodiode detector of claim 14 wherein the atleast one cross-talk blocking layer comprises a metal selected from thegroup consisting of Ti, and Cr.
 16. The avalanche photodiode detector ofclaim 1 wherein the at least one cross-talk blocking layer comprises anindirect-band gap semiconductor.
 17. The avalanche photodiode detectorof claim 16 wherein the at least one cross-talk blocking layer comprisesGe.
 18. The avalanche photodiode detector of claim 1 wherein the atleast one cross-talk blocking layer comprises a metal layer and asemiconductor layer.
 19. The avalanche photodiode detector of claim 1wherein the at least one cross-talk blocking layer is disposed in a bulkregion of the substrate.
 20. The avalanche photodiode detector of claim19 wherein each cross-talk blocking layer in the substrate is separatedfrom other cross-talk blocking layers by substrate bulk regions.
 21. Theavalanche photodiode detector of claim 19 wherein each cross-talkblocking layer in the substrate comprises a semiconductor layer.
 22. Theavalanche photodiode detector of claim 19 further comprising at leastone cross-talk blocking surface layer disposed on the optical interfacesurface of the substrate.
 23. The avalanche photodiode detector of claim19 further comprising a microlens array disposed on the opticalinterface surface of the substrate.
 24. An avalanche photodiode detectorcomprising: a substrate including an array of avalanche photodiodes; anoptical interface surface of the substrate arranged for acceptingexternal input radiation; and at least one cross-talk blocking layer ofmaterial, disposed on the optical interface surface of the substrate,that allows external input radiation to reach photodiodes and thatattenuates radiation in the substrate that is produced by photodiodes inthe array.
 25. The avalanche photodiode detector of claim 24 wherein theat least one cross-talk blocking layer comprises an indirect-band gapsemiconductor.
 26. The avalanche photodiode detector of claim 25 whereinthe at least one cross-talk blocking layer comprises Ge.
 27. Anavalanche photodiode detector comprising: a substrate including an arrayof avalanche photodiodes, each avalanche photodiode provided as astructure of photodiode layers; an optical interface surface of thesubstrate arranged for accepting external input radiation; and at leastone cross-talk filter layer of material disposed in the substrateadjacent to the photodiode structures and including a material thatabsorbs radiation in the substrate that is produced by photodiodes inthe array.
 28. The avalanche photodiode detector of claim 27 wherein thecross-talk filter layer is a semiconductor layer characterized by a bandgap that corresponds to wavelengths of photons produced by photodiodesin the array.
 29. The avalanche photodiode detector of claim 27 whereinthe photodiode layers comprise a semiconducting input radiation absorberlayer and a semiconducting avalanche multiplier layer.
 30. The avalanchephotodiode detector of claim 29 wherein the cross-talk filter layer ischaracterized by a semiconducting band gap corresponding to wavelengthsof radiation produced at the avalanche multiplier layer of photodiodesin the array.
 31. The avalanche photodiode detector of claim 29 whereinthe absorber layer is characterized by a semiconducting band gapcorresponding to wavelengths of external input radiation to be detectedby the photodiode array.
 32. The avalanche photodiode detector of claim27 wherein the absorber layer is characterized by a semiconducting bandgap that sets a long wavelength boundary on a pass band of radiationwavelengths to be detected by the photodiode array and wherein thecross-talk filter layer is characterized by a semiconducting band gapthat sets a short wavelength boundary on a pass band of radiationwavelengths to be detected by the photodiode array.
 33. The avalanchephotodiode detector of claim 27 wherein the substrate comprises InP andthe cross-talk filter layer comprises an InGaAsP alloy.
 34. Theavalanche photodiode detector of claim 33 wherein the absorber layercomprises an InGaAsP alloy.
 35. The avalanche photodiode detector ofclaim 27 wherein the photodiode structures comprise mesa structuresincluding a semiconducting input radiation absorber layer and asemiconducting avalanche multiplier layer.
 36. The avalanche photodiodedetector of claim 27 wherein the cross-talk filter layer is continuousacross the array of photodiodes.
 37. The avalanche photodiode detectorof claim 27 further comprising a microlens array disposed at a positionrelative to the substrate to focus external input radiation to thephotodiodes.
 38. The avalanche photodiode detector of claim 37 whereinthe microlens array is adjacent to and separated from the substrate. 39.The avalanche photodiode detector of claim 37 wherein the microlensarray is disposed on the optical interface surface of the substrate. 40.The avalanche photodiode detector of claim 27 further comprisingconnections from the substrate to photodiode control and read outcircuitry.
 41. The avalanche photodiode detector of claim 27 furthercomprising at least one cross-talk blocking layer of material, disposedon the optical interface surface of the substrate, that allows externalinput radiation to reach photodiodes and that attenuates radiation inthe substrate that is produced by photodiodes in the array.
 42. Theavalanche photodiode detector of claim 27 further comprising at leastone cross-talk blocking layer of material including apertures positionedfor allowing external input radiation to reach photodiodes and includingmaterial regions positioned for attenuating radiation in the substratethat is produced by photodiodes in the array.
 43. The avalanchephotodiode detector of claim 42 wherein the at least one cross-talkblocking layer comprises at least one cross-talk blocking surface layerdisposed on the optical interface surface of the substrate.
 44. Theavalanche photodiode detector of claim 42 wherein the at least onecross-talk blocking layer is disposed in a bulk region of the substrate.